Today a separated heavy phase is removed by                a) discharge of the heavy phase through nozzles in the rotor wall;        b) draining the heavy phase during operation through a valve that is opened and closed;        c) stopping the operation of the separator and removing the heavy phase either by opening the separator or draining the heavy phase.        
Independently of which method used there is always a common problem of when to remove the heavy phase fluid or sludge. With experience it may perhaps be possible to guess, but it may be difficult to decide, especially if the content of heavy phase varies with time.
Methods for detecting a suitable moment for removal of the heavy phase during operation are disclosed, such as in U.S. Pat. No. 3,408,001 where a separator is described having a sludge displacing body arranged inside the sludge space of the rotor to provide a change of the unbalance of the rotor when the heavy phase interface reaches the body.
The change in the condition of balance of a centrifuge rotor, which indicates a suitable time for sludge discharge, can be determined in several different ways. For example, it may be determined by an experienced operator who listens to the sound emitted from the rotating rotor and who initiates the sludge discharge when he detects a familiar change in the sound or vibrations caused by changes in the unbalance.
Other methods for determining this moment may include so called influences, which are relations between the unbalance situation of the separator rotor and the frame vibrations.
To obtain a good view over how a particular separator behaves under different operational conditions it is helpful to map the influences at different rotational speeds and unbalances. When the influences are known they can be used to recognize and determine the changes of unbalances of the sorts mention above.
When this unbalance has reached a predetermined value sludge discharge is triggered.
The prior art provides an apparatus that tries to give information concerning the heavy phase content of the separating space. However, the change in unbalance may often be difficult to detect and interpret due to different operational conditions as it will vary with the fluid mixture to be separated. Also due to the influences being dependent on operational conditions such as temperature, aging or relative movements of components of the separator, the properties of which components therefore change, it is rather difficult to detect a one off change in the vibrations of the separator. The apparatus disclosed in the prior art only provides a change from one unbalance condition to another thus making it easy to miss or misinterpret the event.